The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. A number of known methods have been developed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several issued patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett and assigned to the assignee of this application. This patent describes the in situ recovery of liquid and gaseous carbonaceous materials from subterranean oil shale deposits by fragmenting oil shale in a subterranean oil shale deposit to form a stationary body of fragmented oil shale within the deposit, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying the hot retorting gases used for converting kerogen contained in the oil shale, as described in the '423 patent, includes the establishment of a combustion zone in the retort and the movement of an oxygen supplying gaseous feed mixture downwardly into the combustion zone to advance the combustion zone downwardly through the retort. In the combustion zone oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and a combustion gas. By the continued introduction of the oxygen supplying gaseous feed mixture downwardly into the combustion zone, the combustion zone is advanced downwardly through the retort.
The combustion gas and the portion of the gaseous feed mixture which does not take part in the combustion process pass through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and a residue product of solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid carbonaceous products, together with water formed during combustion, are collected at the bottom of the retort. An off gas containing combustion gases generated in the combustion zone, product gas produced in the retorting zone, gas from carbonate decomposition, and gaseous feed mixture which does not take part in the combustion process is also collected at the bottom of the retort.
The off gas, which contains nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, methane and other hydrocarbons, and sulfur compounds such as hydrogen sulfide, can be used as a fuel or otherwise disposed of but should first be purged of the sulfur compounds. The sulfur compounds in the off gas are generated from naturally occurring sulfur compounds in oil shale during the heating and combustion in the in situ oil shale retort. Unless removed, the sulfur compounds are oxidized to form sulfur dioxide when the off gas is oxidized. Sulfur dioxide is a pollutant and can combine with water vapor in the off gas to form H.sub.2 SO.sub.3 and other polythionic acids which are toxic and corrosive.
While various processes for the removal of sulfur dioxide from gases such as off gas from oil shale retorting have been devised, most such known processes involve contacting the gas with an absorbing agent to convert the sulfur dioxide to a removable liquid or solid. The spent absorbing agent must then either be chemically regenerated or disposed of and replaced. Various absorption agents have been used, such as alkali metal carbonates, but the regeneration rate of these agents is low and the initial cost of many of these agents is too large to permit discharging of the spent agent. Water and limestone have been used as throwaway agents. Water systems have the disadvantage that they require cooling and heating of large quantities of gas and the resulting acidity of the water represents a disposal problem. Lime and limestone have been used as absorbents in both dry systems and wet systems. Since sulfur dioxide reacts more readily with lime, which is calcium oxide, than with limestone, which is principally calcium carbonate, calcination of the limestone is usually used. However, the reaction rate is still prohibitively low at reasonable temperatures so the gas is heated to temperatures above 1000.degree. F. to be effective. A large excess of lime or limestone is required because the resulting calcium sulfite forms on the particle surfaces, thereby quickly reducing the reaction rate with the coated lime or limestone particles.
Thus, there is a need for an economical process for removing sulfur dioxide from the off gas from an in situ oil shale retort.